Optical disk are a safe choice if we need any data to withstand an EMP (Electro Magnetic Pulse) attack. And blu-ray disks are cost effective when comparing it with magnetic storage and shields combined. Say for 1 TB external hard disk drive, current approximate price is USD 50. A 5-pack 25GB recordable blu-ray disc costs around USD 15 which is more that 1 TB storage. Even accounting for the initial price of the disc burner and software, blu-rays are cost effective as we archive more data. Now getting rest of the gear to work after an EMP attack is a different matter, however, the data is preserved.